Optical storage in the compact disc (CD) format has gained wide use in the computer, publishing and consumer markets as a low cost and convenient method of storing information. Digital information is stored in a substrate by means of pits and grooves which are read out with a laser light source. As the technology of optical storage advances to the Digital Versatile Disc (DVD), the density of information on the substrate will need to be increased by making the pits even smaller and closer to one another. Erasable CD's will also create more demands on the resins used in the CDs. Therefore, improvements are needed in the resins and in the manufacturing processes for making CD's and DVD's.
In CD and DVD manufacturing processes, information is encoded in the form of pits and grooves that are replicated from a stamper in a mold onto a plastic substrate. The substrate is then metallized so that the readout laser is reflected from the encoded pits. The CD, after stamping and metallization, is coated with a lacquer or resin which protects the disc and which can be marked with ink for labeling and identification. DVD's are similar to CD's in manufacture, but may also have an added semireflecting layer to encode an additional layer of information on top of the first (replicated) surface. For DVD's, two discs are bonded together with a transparent adhesive to double the information content. The DVD disc is then read from the top and bottom using two light sources.
In the case of recordable CD's and DVD's, a continuous groove or track is molded into the substrate for laser tracking. Then, a phase change material (a dye solution or an inorganic alloy) is spin coated or deposited onto the plastic substrate. A metal alloy, preferably gold, is deposited onto the recording media to reflect the reading laser beam. Finally, a coating is applied to protect the metal layer.
The molding of CD's with fast cycle times requires a plastic resin with a special combination of Theological, mechanical, thermal, water absorption, and optical characteristics. One of the most important properties is a low melt viscosity, which may be achieved by reducing the molecular weight of the polymer. Low melt viscosity enables fast injection molding and also fast relaxation of the birefringence and mechanical stresses in the molded disc before the polymer solidifies. Mechanical properties such as stiffness, shrinkage and impact resistance are important to obtain molded discs that are flat, that have good pit replication, and that are mechanically robust. Resistance to heat is desirable because compact discs are used and stored in uncontrolled environments, such as the inside of an automobile, where temperatures can be very high. Low water absorption is important so that the molded discs retain their performance characteristics in high humidity and temperature. Good adhesion to metal in high humidity is important for long term information storage capability. The optical characteristics of the plastic material include a low stress optic coefficient and high optical transmission so that the laser beam passes through the substrate and is reflected with minimum distortion. The principal performance standards that compact discs must meet are related to fidelity of pit replication (information content), low birefringence in substrate (optical signal fidelity), and flatness of disc (no wobble in rotating disc).
It is difficult for a plastic resin to meet all of the above requirements. For example, polymers with melt viscosity that has been reduced by lowering the molecular weight generally have poor mechanical properties. A number of plastics have been proposed or used, including polycarbonates, acrylics, polyesters, polystyrene and cyclic olefin copolymers. Bisphenol-A polycarbonate and acrylic polymers are the most widely used, mainly because of their high optical transmission, excellent mechanical properties (polycarbonate) and low birefringence (acrylics). However, these materials have a number of disadvantages, including a high stress optic coefficient (polycarbonate), high water absorption (acrylic), and poor metallization (acrylic). There is therefore a need for an improved optical quality plastic resin as a substrate material for compact discs.
Plastics that can be used in CD's and DVD's ("optical plastics") have sufficiently good optical properties so that they can be used in other optical articles as well. The term, "optical articles" as used herein refers to articles that are used in optical applications. Optical articles deal with light, including the effects that the light produces and undergoes, and are used in the generation, propagation, and/or transmission of light. "Optical plastics" are plastics that have optical properties that make them suitable for use in optical articles. Optical articles include lenses, such as Fresnel lenses, instrument panel windows and covers, prismatic reflectors, optical fibers, CD's, DVD's, and transparent sheets and films. Molded lenses can be used for such applications as the directing of light beams and images. Plastic lenses are useful because additional mechanical features and fittings can easily be incorporated into the lens design and because they are easy to produce by molding. The materials requirements for lenses are similar to those of compact discs.
The diol 1,4:3,6-dianhydro-D-sorbitol, referred to hereinafter as isosorbide, the structure of which is illustrated below, is readily made from renewable resources, such as sugars and starches. For example, isosorbide can be made from D-glucose by hydrogenation followed by acid-catalyzed dehydration. ##STR1##
Isosorbide has been incorporated as a monomer into polyesters that also include terephthaloyl moieties. See, for example, R. Storbeck et al, Makromol.Chem., Vol. 194, pp. 53-64 (1993); R. Storbeck et al, Polymer, Vol. 34, p. 5003 (1993). However, it is generally believed that secondary alcohols such as isosorbide have poor reactivity and are sensitive to acid-catalyzed reactions. See, for example, D. Braun et al., J. Prakt.Chem., Vol. 334, pp. 298-310 (1992). As a result of the poor reactivity, polyesters made with an isosorbide monomer and esters of terephthalic acid are expected to have a relatively low molecular weight. Ballauff et al, Polyesters (Derived from Renewable Sources), Polymeric Materials Encyclopedia, Vol. 8, p. 5892 (1996).
Copolymers containing isosorbide moieties, ethylene glycol moieties, and terephthaloyl moieties have been reported only rarely. A copolymer containing these three moieties, in which the mole ratio of ethylene glycol to isosorbide was about 90:10, was reported in published German Patent Application No. 1,263,981 (1968). The polymer was used as a minor component (about 10%) of a blend with polypropylene to improve the dyeability of polypropylene fiber. It was made by melt polymerization of dimethyl terephthalate, ethylene glycol, and isosorbide, but the conditions, which were described only in general terms in the publication, would not have given a polymer having a high molecular weight.
Copolymers of these same three monomers were described again recently, where it was observed that the glass transition temperature Tg of the copolymer increases with isosorbide monomer content up to about 200.degree. C. for the isosorbide terephthalate homopolymer. The polymer samples were made by reacting terephthaloyl dichloride in solution with the diol monomers. This method yielded a copolymer with a molecular weight that is apparently higher than was obtained in the German Patent Application described above but still relatively low when compared against other polyester polymers and copolymers. Further, these polymers were made by solution polymerization and were thus free of diethylene glycol moieties as a product of polymerization. See R. Storbeck, Dissertation, Universitat Karlsruhe (1994); R. Storbeck, et al., J. Appl. Polymer Science, Vol. 59, pp. 1199-1202 (1996).
U.S. Pat. No. 4,418,174 describes a process for the preparation of polyesters useful as raw materials in the production of aqueous stoving lacquers. The polyesters are prepared with an alcohol and an acid. One of the many preferred alcohols is dianhydrosorbitol. However, the average molecular weight of the polyesters is from 1,000 to 10,000, and no polyester actually containing a dianhydrosorbitol moiety was made.
U.S. Pat. No. 5,179,143 describes a process for the preparation of compression molded materials. Also, described therein are hydroxyl containing polyesters. These hydroxyl containing polyesters are listed to include polyhydric alcohols, including 1,4:3,6-dianhydrosorbitol. Again, however, the highest molecular weights reported are relatively low, i.e. 400 to 10,000, and no polyester actually containing the 1,4:3,6-dianhydrosorbitol moiety was made.
Published PCT Applications WO 97/14739 and WO 96/25449 describe cholesteric and nematic liquid crystalline polyesters that include isosorbide moieties as monomer units. Such polyesters have relatively low molecular weights and are not isotropic.